1. Field of the Invention
This invention relates to pnuumatic tires, and more particularly to a pneumatic tire having improved heat resistance and gripping performance.
2. Related Art Statement
Recently, the use environment of rubber articles has become increasingly severe. Particularly, this tendency has become conspicuous in automobile tires accompanied with the improvement of performances of the automobile, the pavement of roads and the completion of expressway networks. As a result, there is presently a strong demand for the development of pneumatic tires having high running performances, particularly high gripping performance. The gripping performance is an important requirement during acceleration, deceleration and cornering performance of the tire. As the gripping performance becomes higher, it is possible to run the tire at a higher speed.
In order to obtain high gripping performance, it has hitherto been required to select rubber having a high glass transition temperature, such as styrene-butadiene copolymer rubber having a high styrene content or the like, as a tread rubber composition for the tire, or to select a rubber composition containing large amounts of carbon black and process oil to increase the dynamic loss factor (tan .delta.) of the rubber composition.
Furthermore, in order to improve the decrease of the gripping performance accompanied with the rising of the temperature during the high speed running, Japanese Patent laid open No. 59-187,011 discloses the use of a copolymer rubber obtained by copolymerizing a monomer such as 1,3-butadiene, styrene, isoprene or the like, with an acrylate or methacrylate compound containing diphenylphosphate group, such as diphenyl-2-methacryloyloxyethyl phosphate, diphenyl-2-acryloyloxyethyl phosphate or the like.
When the styrene content in the styrene-butadiene copolymer as the tread rubber composition for providing the high gripping performance increases, however, the gripping performance is certainly improved temporarily, but the value of tan .delta. undesirably lowers in attendance with the rising of the rubber temperature during the running as a result the gripping performance rapidly decreases.
On the other hand, when selecting a rubber composition containing large amounts of carbon black and process oil, the gripping performance is improved, but the fracture properties and wear resistance are considerably lower due to the addition of a large amount of the additive.
Furthermore, the method of preventing the degradation of the gripping performance with the temperature rising as disclosed in Japanese Patent laid open No. 59-187,011 can not be applied to natural rubber and also has a problem that the properties inherent to the polymer, such as styrene-butadiene copolymer rubber, polybutadiene rubber or the like, are damaged in accordance with the production conditions.